Drill strings are typically constructed of short, individual sections of drill pipes or rods. The drill rods attach to one another to form a drill string, which can extend significant distances in some drilling applications. The drill rods used in small to medium sized horizontal drilling machines are typically either ten feet or fifteen feet in length. A drill string often extends over one hundred to three hundred feet in length. Thus, it is not unusual for a drill string to be assembled using 10 to 30 sections of drill rods, or more.
Referring now to FIG. 1, one known drill rod assembly 10 used in conventional drilling systems is illustrated. The drill rod assembly 10 includes an outer tubular drill rod 30 having external threads on one end and internal threads on the opposite end. The drill rod assembly 10 further includes a smaller, inner drill rod 20. The inner drill rod 20 fits inside the tubular outer rod 30. As previously described, typical drill rods are either ten feet or fifteen feet in length. Drill rod assemblies having inner and outer rods, however, are uncharacteristically short, to address stack-up problems described in greater detail hereinafter. The illustrated drill rod assembly 10 is only three feet in length.
Drill rods are typically positioned in the drilling machine, with one end higher than the other; thus, the illustrated assembly 10 has an up-hill end 36 and a down-hill end 38, as shown. The inner drill rod 20 includes a hexagonal first end 29 and a hexagonal second end 27. A coupling 22 is affixed to the first end 29 by a cross pin 26 that passes through a hole 25 formed in the inner drill rod 20. The cross pin 26 has an interference fit such that the pin 26 remains fixed within the hole 25 of the inner drill rod 20 when properly installed. The cross pin 26 also passes through a slotted hole 23 formed in the coupling 22. The coupling 22 has a larger diameter D1 than that of an inner diameter ID1 of the outer drill rod 30 at the up-hill end 36 of the assembly 10. The larger outer diameter OD1 of the coupling 22 prevents the inner drill rod 20 from sliding through the outer drill rod 30. The inner drill rod 20 also includes an enlarged portion 28 located adjacent to the down-hill end 38 of the assembly 10. The enlarged portion 28 prevents the inner drill rod 20 from sliding through the outer drill rod 30 in an opposite direction.
The drill rod assembly 10 is constructed by installing the inner drill rod 20 into the outer drill rod 30 at the down-hill end 38 of the assembly 10. In particular, the inner drill rod 20 is installed within the outer drill rod 20 until the expanded portion 28 of the inner drill rod 10 contacts the outer drill rod 30, and limits longitudinal movement; or until the hole 25 of the inner drill rod 20 aligns with the slotted hole 23 of the coupling 22, so that the cross pin 26 can be inserted. The coupling 22 includes an internal hexagonal bore that mates with the hexagonal first end 27 of the inner drill rod to fix the coupling and inner drill rod rotationally. The mating hexagonal bore and the hexagonal first end 29 of the coupling and inner drill rod transmit torque, while the cross pin 26 simply holds the coupling 22 and the rod 20 in place.
When assembled, the inner drill rod assembly 20 freely moves in a longitudinal direction from the position illustrated in FIG. 1 to a position where the enlarged portion 28 of the inner drill rod 20 contacts the outer drill rod 30. That is, the inner drill rod 20 slides longitudinally between an up-hill position and a down-hill position. In the up-hill position, a gap is formed between the coupling 22 and the outer drill rod 30 at the up-hill end 36 of the assembly 10. In the down-hill position, the coupling 22 is flush with the outer drill rod 30 at the up-hill end 36 of the assembly.
FIG. 2 illustrates the drill rod assembly 10 coupled to a boring tool 40. The boring tool 40 is connected to the down-hill end 38 of the assembly 10. The boring tool 40 includes an outer casing 45 having an external threaded end 44. The boring tool 40 also includes an inner rod 42 and an attached coupling 43 having an internal hexagonal bore. Unlike the drill rod assembly 10, however, the inner rod 42 of the boring tool 40 is coupled to the outer casing 45 in a fixed position. That is, the inner rod 42 of the boring tool 40 does not longitudinally slide relative to the outer casing 45. Accordingly, when the drill rod assembly 10 is coupled to the boring tool 40, the fixed position of the inner rod 42 of the boring tool 40 determines the position of the inner drill rod 20 of the drill rod assembly 10 relative to the outer drill rod 30.
More specifically, when the drill rod assembly 10 is threaded onto the boring tool 40, the coupling 43 of the inner rod 42 engages with the second hexagonal end 27 of inner drill rod 20. The inner drill rod 20 is normally positioned as shown in FIG. 1 by gravity; i.e., positioned such that the coupling 22 is flush with the outer drill rod 30 at the up-hill end 36 of the assembly 10. As the assembly 10 threads onto the boring tool 40, the inner drill rod 20 of the assembly 10 is pushed or slides longitudinally toward the up-hill end 36 of the assembly. The inner drill rod 20 slides such that an axial gap 100 is created between the coupling 22 and the outer drill rod 30, as depicted in FIG. 2. In operation, the axial gap 100 serves as a fluid flow path that allows fluid to enter the drill rod assembly 10 and pass through an annular area between the inner and outer drill rods 20, 30. From the annular area of the assembly 10, the fluid passes through to the boring tool 40 to cool the boring tool and assist in the transportation of cuttings.
FIG. 3 illustrates first and second drill rod assemblies 10a and 10b connected to form a drill string. The same boring tool 40 is coupled to the down-hill end of the drill string (i.e., the down-hill end of the lowermost drill rod assembly 10a). The first drill rod assembly 10a is connected to the second drill rod assembly 10b by threading an externally threaded up-hill end 32a of the first outer drill rod 30a into an internally threaded down-hill end 34b of the second outer drill rod 30b. As the outer drill rods 30a, 30b are being coupled, the coupling 22a of the first inner drill rod 20a engages the hexagonal end 27b of the second inner drill rod 20b. 
The drill string defines a fluid flow path that extends along the lengths of the drill rod assemblies 10a, 10b. In operation, fluid is pumped into the upper most drill rod assembly, through the fluid flow path, and into the boring tool for cooling and transporting cuttings. For example, referring specifically to FIG. 3, fluid is pumped into the annular area between the inner and outer drill rods 20b, 30b of the second drill rod assembly 10b, through the gap 100 of the first drill rod assembly 10a, then through the annular area between the inner and outer drill rods 20a, 30a of the first drill rod assembly 10a, and into the boring tool 40.
As previously described, the fixed position of the inner rod 42 of the boring tool 40 determines the position of the inner rod 20a of the first drill rod assembly 10a. That is, the position of the inner drill rod 20a becomes fixed relative to the outer drill rod 30a when attached to the boring tool 40. The now fixed positions of the first inner and outer drill rods 20a, 30a of the first drill rod assembly 10a accordingly determine the position of the second inner drill rod 20b relative to the second outer drill rod 30b of the second drill rod assembly 10b. As the second assembly 10b threads onto the first assembly 10a, the second inner drill rod 20b is pushed or slides longitudinally such that an axial gap 102 is created between the coupling 22b and the second outer drill rod 30b, as depicted in FIG. 3. Fluid now enters the drill string at the axial gap 102 of the second drill rod assembly 10b, passes through to the first drill rod assembly 10a, and further passes through to the boring tool 40 to cool the boring tool and assist in the transportation of cuttings.
The inner and outer drill rods 20, 30 of each of the drill rod assemblies 10a, 10b have unavoidable variations in length resulting from manufacturing tolerances. Because of the length variations, drill rod assemblies are designed such that the overall length of interconnected inner drill rods 20a, 20b is never longer than the overall length of interconnected outer drill rods 30a, 30b. If the interconnected inner drill rods were longer than the outer drill rods, the inner rods would collide while the outer drill rods were being threaded together, causing damage to one or both of the inner and outer drill rods. Accordingly, by design, the length of interconnected inner drill rods is slightly less than the length of interconnected outer drill rods. This design requirement, however, results in a situation where the second axial gap (e.g., 102) of an up-hill drill rod assembly (e.g., 10b) is less than the first axial gap (e.g., 100) of a down-hill drill rod assembly (e.g., 10a).
FIG. 4 illustrates a drill string with a boring tool 40 and four drill rod assemblies 10a, 10b, 10c, and 10d. The difference in the overall lengths of the interconnected inner and outer drill rods, and the manufacturing variations of the drill rods, are depicted in an exaggerated manner to better illustrate the effect of this design limitation.
FIG. 4a illustrates the first axial gap 100 defined by the position of the first coupling 22a relative to the outer drill rod 30a of the first drill rod assembly 10a. When the second drill rod assembly 10b is coupled to the first assembly 10a, the first end 29a of the first inner drill rod 20a contacts the second end 27b of the second inner drill rod 20b of the second assembly 10b, and determines the relative positions of the second inner and outer drill rods 20b, 30b. 
FIG. 4b illustrates the second axial gap 102 defined by the position of the second coupling 22b relative to the outer drill rod 30b of the second drill rod assembly 10b. The axial gap 102 is smaller than the first axial gap 100. When the third drill rod assembly 10c is coupled to the second assembly 10b, the first end 29b of the second inner drill rod 20b contacts the second end 27c of the third inner drill rod 10c of the third assembly 10c, and determines the relative positions of the third inner and outer drill rods 20c, 30c. 
FIG. 4c illustrates the position of the coupling 22c of the third drill rod assembly 10c relative to the third outer drill rod 30c. There is no gap (shown at arrow 104). Instead, the coupling 22c is seated against the up-hill end 32c of the third outer drill rod 30c. When the fourth drill rod assembly 10d is coupled to the third assembly 10c, the first end 29c of the third inner drill rod 20c is spaced apart from the second end 27d of the fourth inner drill rod 20d. The space between these ends 29c, 27d of the inner drill rods 20c, 20d is caused by the fact that the coupler 22d (FIG. 4d) of the fourth assembly 10d has contacted the uphill end 32d of the outer drill rod 30d; thereby positioning the fourth inner drill rod 20d relative to the outer drill rod 30d. That is, the inner drill rod 20d can no longer shift or slide down longitudinally toward the down-hill end of the assembly, but is instead stopped by contact between the coupling 22d and the outer drill rod 30d. 
Because of the design requirement that the inner rods always be shorter than the outer rods, any drill rod assemblies subsequently added to the fourth drill rod assembly 10d will have inner and outer drill rods similarly positioned as shown in FIG. 4d. That is, the couplings 22 of subsequently added drill rod assemblies 10 will be in contact with the outer drill rods 30, such that no gaps exist in the drill string. This results in a blockage of the fluid flow path of the drill string. Such blockages are a known problem in the industry.
In view of the foregoing, there exists a need for a drill rod assembly, having inner and outer coaxial drill rods, that minimizes and/or eliminates restricted fluid flow paths upon assembly into a drill string.